In the investigation of muscle problems, it is relatively easy to measure the circulating levels of nutrients known to be involved in muscle function. Such data frequently fail to provide enough information about metabolic problems within the muscle. In diagnosis, and in following the treatment of muscle-related problems, a functional test is needed.
A helpful description of the theoretical and physiological aspects of muscle function is given by Poland et al (1981) The Musculoskeletal System, 2nd Edn., pp. 8-73, London: Henry Kimpton.